Our interests continue to involve the study of the phagocytic function of the reticuloendothelial cells and circulating leukocytes. Ongoing studies include the evaluation of the role of antibody in acquired neutropenia, the attempt to develop a suitable small biodegradable human serum albumin microsphere for the study of the reticuloendothelial system in man. In addition, in view of the recent reports advocating ionic indium-lll as a bone marrow scanning agent with the assumption that its localization reflects the erythropoietic status of the patient, we have resumed our previous studies on the physiological behavior of indium and iron bound to transferrin, particularly in reference to the hematopoietic and reticuloendothelial system.